[Dbix-class] Unexpected duplicate-key error from find_or_create

[Lee Goddard]

I do not often find myself using DBIx::Class, so please forgive any 
naivety or silliness in my question, but I would appreciate your help.

   my $o = $schema->resultset( $field )->find_or_create(
     value => $val
   );

The above code has been working well over a variety of data, until just 
now, when it died with the following words (where `331_chrs` represents 
a string of 331 characters):

SELECT me.id, me.value FROM licence me WHERE ( ( me.value = ? ) ): 
'331_chrs'
INSERT INTO licence (value) VALUES (?): '331_chrs'
DBIx::Class::ResultSet::find_or_create():
   DBI Exception:
   DBD::mysql::st execute failed:
   Duplicate entry '%s' for key 2
   [for Statement "INSERT INTO licence (value) VALUES (?)"] at ...

I have not yet taken to reading the source code, because I this is the 
one error I had hoped to avoid by using find_or_create.

The MySQL table in question has only two columns, one PRIMARY and one 
UNIQUE, and I'm developing with the fantastic DBIx::Class::Schema::Loader.

Have you seen such behaviour before? I am making a common error? Have I 
misconstrued something in the manual? Should I be reading the source? 
Could this be DBIx::Class::Schema::Loader...?

Thanks in anticipation of any pointers,
Lee